A common form of linear speed control for a moving web as used, for example, in tape and ribbon transports, employs a constant speed capstan pinch roller drive. Where constant ribbon velocity is required in such transports, a special drive is required for the takeup reel and the payout reel as well as for the pinch roller in order to attain reasonable precision in the control of the surface velocity of the tape. Added complications arise when the web has to be driven bidirectionally at high speed. Heretofore, the use of a pinch roller has provided certain disadvantages, as, for example, problems in maintaining the proper ribbon tension during ribbon movement, and improper tracking of the ribbon with respect to the head (particularly where the ribbon has substantial width).
Proper ribbon tension in such tape and ribbon transports is often critical. For example, in an impact printer with a moving print head having an inked ribbon suspended between the print head and a record medium (such as paper) by two guides (located on either side of the printer), the ribbon must be suspended at a proper constant tension. Insufficient ribbon tension may result in the ribbon being caught in the print wires of the print head and dragged along with the head. Excessive ribbon tension, on the other hand, can cause stalling of the ribbon take-up spool, curling or improper winding of the ribbon onto the take-up spool, or breakage of the ribbon.
Friction guides have sometimes been used in the past to provide required ribbon tension. Friction guides, however, have several disadvantages. For example, friction guides tend to collect dried ink, causing the ribbon to adhere to the guides; ribbon tension varies with friction guide wear; and the friction guides can interfere with the correct spooling of the ribbon onto the take-up spool.
Now, however, I have discovered an improved control for providing constant ribbon tension as well as constant surface velocity of a moving web or tape. As implemented in the exemplary embodiment, it requires a lesser number of mechanical components to provide simultaneous linear velocity and tension control. This exemplary embodiment is, for example, useful in providing a linear velocity and tension control for use with an inked ribbon having substantial width. And it may provide constant ribbon tension for a moving ribbon suspended between a print head and a record medium.